Human biotinidase (BTD, EC 126.96.36.199) (Cole et al. 1994) catalyzes the hydrolysis of biocytin (BCTN, aka biotinyl-lysine), a product of biotin dependent carboxylase degradation, to biotin (Btn) and lysine. As a result, Btn is again available to be used in the biotinylation of apo-carboxylases in the mitochondrion. BTD is both secreted from various cells and localised in the mitochondria (Wolf & Jensen 2005). BTD deficiency, an autosomal recessive disorder, results in a secondary Btn deficiency that leads to juvenile onset multiple carboxylase deficiency (MIM:253260) (Wolf et al. 1983).